1. Field of the Invention
The present invention relates to a magnetooptical information recording and/or reproducing apparatus for recording information on a magnetooptical recording medium and/or reproducing the recorded information from the magnetooptical recording medium.
2. Related Background Art
In recent years, a magnetooptical information recording/reproducing apparatus using a magnetooptical disk as a recording medium has received a great deal of attention as the most promising recording/reproducing apparatus because it is portable, has a large storage capacity, and is capable of performing information erasure and updating. FIG. 1 is a view showing an optical system of a conventional magnetooptical information recording/reproducing apparatus. This apparatus includes a semiconductor laser 19 serving as a recording/reproduction light source. A divergent light beam emitted from the semiconductor laser 19 is collimated by a collimator lens 20. The sectional shape of the collimated light beam is shaped into a circular shape by a beam shaping prism 21. The collimated light beam is a beam of linear polarization (to be referred to as a P-polarized light component) having a polarization direction parallel to the drawing surface. The P-polarized light beam is incident on a polarizing beam splitter 22. The characteristics of the polarizing beam splitter 22 are given such that the transmittance for the P-polarized light component is 60%, the reflectance therefor is 40%, the transmittance for a linearly polarized light component (to be referred to as an S-polarized light component) having a polarization direction perpendicular to the polarization direction of the P-polarized light component is 0%, and the reflectance for the S-polarized light component is 100%. The P-polarized light beam passing through-the polarizing beam splitter 22 is focused by an objective lens 23, so that a light spot is formed on the magnetic layer of a magnetooptical disk 24. An external magnetic field is applied from a magnetic head 25 to a portion irradiated with this light spot, thereby recording an information domain on the magnetic layer.
Light reflected by the magnetooptical disk 24 returns to the polarizing beam splitter 22 through the objective lens 23. Part of the reflected light is split and guided to a reproduction optical system. In the reproduction optical system, the split light beam is further split by another polarizing beam splitter 26. The characteristics of the polarizing beam splitter 26 are given such that the transmittance for the P-polarized light component is 20%, the reflectance for the P-polarized light component is 80%, the transmittance for the S-polarized light component is 0%, and the reflectance for the S-polarized light component is 100%. One light beam split by the polarizing beam splitter 26 is guided to a reproduction optical system 27, and a reproduction signal (to be described later) is generated. The other light beam is guided to a half prism 36 through a condenser lens 35. This light beam is split into halves by the half prism 36. One light beam is guided to a photodetector 37, and the other light beam is guided to a photodetector 39 through a knife edge 38. By these control optical systems, servo error signals for autotracking control and autofocusing control of the light beam are generated.
The reproduction optical system 27 comprises a .lambda./2 plate 28 for rotating the polarization direction of the light beam through 45.degree., a condenser lens 29 for condensing the light beam, a polarizing beam splitter 30, and photodetectors 31 and 32 for respectively detecting the light beams split by the polarizing beam splitter 30. The characteristics of the polarizing beam splitter 30 are given such that the transmittance for the P-polarized light component is 100%, the reflectance for the P-polarized light component is 0%, the transmittance for the S-polarized light component is 0%, and the reflectance for the S-polarized light component is 100%. Signals detected by the photodetectors 31 and 32 are differentially detected by a differential amplifier 33 to generate a reproduction signal 34. In the magnetooptical recording medium, information is recorded in accordance with a difference in direction of perpendicular magnetization. Recording schemes are classified into an optical modulation scheme and a magnetic field modulation scheme. The optical modulation scheme is a scheme for radiating a laser beam intensity-modulated in accordance with recording information on a recording medium to record the information while a predetermined external magnetic field is being applied to the recording medium. On the other hand, the magnetic field modulation scheme is a scheme for applying an external magnetic field modulated in accordance with recording information while a recording medium is being irradiated with a laser beam having a predetermined intensity.
When a linearly polarized light component is radiated on a magnetooptical recording medium on which information is recorded depending on a difference in direction of magnetization, the polarization direction of the reflected light is rotated clockwise or counterclockwise depending on the difference in direction of magnetization. For example, as shown in FIG. 2, assume that the polarization direction of the linearly polarized light component incident on the magnetooptical recording medium is a P (ordinate) direction, that reflected light with respect to downward magnetization is R.sub.+ rotated through +.theta..sub.k, and that reflected light with respect to upward magnetization is R.sub.- rotated through -.theta..sub.k. When an analyzer is located in a direction indicated in FIG. 2, light passing through the analyzer becomes A with respect to R.sub.+ and B with respect to R.sub.-. When these light components are detected by the photodetectors, information as a difference in light intensity can be obtained. In the example of FIG. 1, the polarizing beam splitter 30 plays a role as the analyzer for rotating one split light beam through +45.degree. from the P-axis and the other light beam through -45.degree. from the P-axis. That is, the resultant signal components from the photodetectors 31 and 32 have opposite phases. When these signals are differentially detected, a reproduction signal whose noise is reduced is obtained. In order to increase the storage capacity, a recording scheme is recently being shifted from a mark interval recording scheme in which the central position of an information pit is meaningful to a mark length recording scheme in which the length of an information pit is meaningful. According to the mark length recording scheme, the position of an information pit is optically detected with an optical head, and the position information is electrically processed to detect the edge of the information pit.
In a conventional technique, however, light in the P-axis direction and light in the S-axis direction are detected using the polarizing beam splitter 30 shown in FIG. 1 to reproduce information. In this reproduction scheme, the number of components of the optical head is increased to undesirably result in a bulky, complicated arrangement. In the mark length recording scheme, the edge of the information pit (domain) must be detected. In the conventional technique, the edge is not directly optically detected. When the size of the domain becomes equal to that of the light spot, the detection position of the edge is shifted, and information cannot be accurately detected, resulting in inconvenience.